The growing demand for liquid fuels and increasingly difficult means of supplying petroleum have led to the interest in biofuels. This umbrella term is applied to any form of energy that is ultimately derived from solar energy via photosynthesis. Despite their many promises, concerns have been raised about biofuels' feasibility, scale, and overall energy balance, as well as many ethical issues surrounding those that are also farmed for food. Many believe that algae represent the best crop for the biofuels of the future.
The events leading from light to liquid fuel can be organized into a series of phases. These phases are growth, separation, extraction, and refinement. The growth of algae can be accomplished through the use of a photobioreactor, or PBR. Photobioreactor is the generic term for the device used to cultivate a photosynthetic organism, such as algae. Once an algaculture has been grown it must be separated from the growth media. There are many means of algae harvest available and with any option there will be a tradeoff between efficiency and energy input. The extraction of hydrocarbons from the harvested algae represents the final phase for the ‘algal farm’. Extracted hydrocarbons can then be packaged and transported to refinement facilities for processing into biodiesel, pyrolytic hydrocracking or any other conversion.
Many different concerns must be addressed when designing a PBR; chief among them is the availability and intensity of light. Light will dictate the size and location of the PBR. The need to introduce adequate levels of CO2 and remove produced O2 presents another set of design restrictions. Beyond these, there are secondary concerns that must be addressed including media selection, materials used, agitation system, and sterility. Open-air PBRs suffer from increased susceptibility to contamination and water loss through evaporation. Closed PBRs not only circumvent these detrimental problems, they also afford the ability to better control the conditions of the algaculture. There are many different designs available for closed PBRs, but the most popular is the tubular PBR. This simple system involves pumping algaculture through a transparent tube that is exposed to sunlight. The tube can be arranged in many different ways to best utilize space, light availability and gas transfer. A common motif with tubular PBRs is to build a ‘fence’ of tubes. Other designs include a figure-eight arrangement and a right triangle where gas pumped at the lower end of the hypotenuse drives the flow of algaculture up the hypotenuse providing circulation. Though these designs have valid concepts, they are lacking in many respects, namely overall algaculture volume, ratio of algaculture volume to material used, ratio of algaculture volume to land area used, gas mass transfer capabilities, and ease of harvest.
In order to address all these issues while still preserving the positive qualities of the tubular PBR, a novel PBR is proposed that has design considerations for every concern addressed.